The present invention generally relates to metal alloys useful for storing hydrogen. More specifically, the invention provides aluminum alloys particularly well suited for hydrogen storage and methods of producing the same.
Petroleum fuels are currently the primary fuels for operating internal combustion engines and turbines in vehicles, generators and many other applications. Gasoline and diesel, for example, are currently the most popular fuels for operating cars, trucks, machinery and other motorized equipment. It is estimated that the transportation sector consumes nearly 50% of the total petroleum fuels consumed in the United States. One problem of using petroleum fuels is that they produce a significant amount of air pollution. Another serious problem of using petroleum fuels is that the United States and other industrialized countries import more than 50% of the oil that they consume. As a result, the economies and the national security of many industrialized countries are susceptible to production controls and foreign policy concerns of foreign petroleum producing countries. Therefore, it is well recognized that there is a high demand for systems that can generate, distribute and use abundant and clean transportation fuels.
Hydrogen is one of the most promising fuels that is being considered to replace petroleum fuels for the transportation sector. In the case of vehicles, hydrogen fuel-cells that generate electricity from a flow of hydrogen are being used to power electric automobile engines, and combustion engines that burn hydrogen are being used in other applications. One advantage of using hydrogen is that it does do not produce air pollution. An advantage of using hydrogen fuel-cells is that vehicles will not need to carry large, heavy batteries to store electrical power because the hydrogen fuel-cells provide a power plant onboard the vehicles. As a result, electrical vehicles with hydrogen fuel-cells are expected to be lighter and more efficient than existing battery-powered electrical vehicles. Hydrogen fuels also provide more energy than either gasoline or natural gas on a per-weight basis, and hydrogen is also readily abundant from resources within the borders of the United States and other industrialized countries. Hydrogen fuels may accordingly reduce the economic and foreign policy concerns caused by importing a significant percentage of the petroleum fuels. Therefore, it would be very beneficial to replace gasoline and diesel with hydrogen as a viable fuel for the transportation sector.
The implementation of a national energy economy based on hydrogen fuels will require the development of many systems and processes to make hydrogen fuels as safe and convenient to use as gasoline or diesel. One area of hydrogen fuel technology that needs further development is storing hydrogen in a safe, efficient manner. Although hydrogen has more energy than gasoline on a per-weight basis, it has a much lower energy/unit volume than gasoline. As a result, conventional hydrogen storage systems require a much larger storage vessel than gasoline tanks to provide the same vehicle operating range, for example. The United States Department of Energy has established energy density goals for storing hydrogen onboard vehicles at 6.5 weight percent H2 and 62 kg H2/m3. Existing storage systems for compressed or liquefied hydrogen are generally high-pressure storage vessels with a vacant cavity that can hold approximately 6.7 weight percent H2 and 20 kg H2/m3 at a pressure of 5000 psi, falling short of the desired density.
Gas-on-solid adsorption offers the possibility of a more dense storage medium than would be achieved utilizing hydrogen alone. While a number of materials have been suggested as viable solid media, more research has probably been done on metal hydrides than most other potential gas-on-solid hydrogen storage media. A number of metal alloys are known to be able to reversibly absorb and release hydrogen. For example, FeTi, LaNi5, Mg2Ni and pure magnesium have all been investigated as hydrogen storage media.
Many of these hydridable metals have some disadvantages, though. For example, FeTi and LaNi5 both have typical hydrogen storage capacities of less than two weight percent hydrogen. Mg2Ni is better, with hydrogen storage capacities of as high as 3.8 weight percent, but this alloy still falls short of the 6.5 weight percent target. Magnesium seems like an excellent candidate in that it has a nominal hydrogen storage capacity of 7.6 weight percent. Unfortunately, though, magnesium oxidizes vary easily. A skin of magnesium oxide tends to form on the metal particles, effectively preventing the particles from absorbing hydrogen except under very high pressures and temperatures.
Another class of metal hydrides that shows significant promise is a class of alloys referred to as xe2x80x9calanates.xe2x80x9d Alanates are all hydridable aluminum alloys and include NaAl, LiAl, MgAl2 and ZrAl2. Each of these alloys has rather high theoretical hydrogen storage capacitiesxe2x80x94NaAlH4 can theoretically hold 5.6 weight percent hydrogen and 56 Kg H2/m3; Mg(AlH4)2 has a potential of storing 6.95 weight percent hydrogen at densities of 70 Kg H2/m3; and LiAlH4 may store as much as 7.9 weight percent hydrogen at densities of 79 Kg H2/m3. The storage capacity of NaAl approaches the U.S. Department of Energy goal while MgAl2 and LiAl both exceed the goal.
Unfortunately, all three of these compounds have relatively slow reaction kinetics for absorbing and desorbing hydrogen. As a consequence, relatively high operating temperatures and pressures are necessary and it takes unduly long for these materials to take up and release significant quantities of hydrogen.
Various aspects of the present invention provides a nanocrystalline powder suitable for storing hydrogen and a method of producing such a powder. One embodiment of the invention provides a nanocrystalline powder which comprises polycrystalline particles containing crystals of an aluminum alloy selected from the group consisting of NaAlx, LiAlx, MgAl2x, and ZrAl2x, wherein x is between 0.9 and 1.1, desirably 0.95-1.05, preferably about 1. The nanocrystalline powder also estimably includes an extracrystalline catalyst present on and in the polycrystalline particles. The catalyst may be selected from the group consisting of carbon, titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, lanthanum, and combinations of two or more of carbon, titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, and lanthanum. The aluminum alloy crystals desirably have a grain size of no more than 100 nm. The catalyst may be intercalated between the crystals of the polycrystalline particles to further facilitate hydrogen uptake and release.
In accordance with another embodiment, the present invention provides a method of forming a hydrogen storage medium. In accordance with one aspect of this invention, a charge is provided, with the charge comprising powdered aluminum and a powder of a second metal in a predetermined atomic ratio. The second metal is selected from the group consisting of lithium; sodium; magnesium; zirconium; alloys of two or more of lithium, sodium, magnesium, and zirconium; and a mechanical mixture of two or more of lithium, sodium, magnesium, and zirconium. This charge is milled, such as by impact milling in a ball mill, to create a nanocrystalline alloy powder comprising crystals of MAlx, wherein x may be close to 1 for each atom of M which is lithium or sodium and x may be close to 2 for each atom of M which is magnesium. In a further embodiment-of the method, a catalyst is intercalated on and into particles of the alloy powder. The intercalated catalyst is selected from the group consisting of carbon, titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, lanthanum, and combinations of two or more of carbon, titanium, platinum, palladium, vanadium, zirconium, cobalt, nickel, iron, and lanthanum. The catalyst can be intercalated by milling the nanocrystalline alloy powder with a powder comprising the catalyst. The catalyst powder may comprise a fullerene structure consisting essentially of carbon.
Another embodiment of the invention provides a metal suitable for reversibly storing hydrogen. The metal may have the formula MAlx-yMxe2x80x2y and can form a hydride having the formula MAlx-yMxe2x80x2yH4. In these formulae:
M comprises a metal selected from the group consisting of sodium, lithium, magnesium, and zirconium;
Mxe2x80x2 comprises an element selected from the group consisting of silicon, potassium, calcium, barium, titanium, lithium, sodium, magnesium, and zirconium;
x is between 0.9 and 1.1 if M is sodium or lithium and x is between 1.8 and 2.2 if M is magnesium or zirconium; and
y is between 0.1 and 0.5.